Coriolis mass flow meters include a measuring tube, which, during a measurement operation, is interposed in the pipeline section, such that the medium flows through it. The measuring tube is caused to oscillate. The oscillation of the measuring tube is affected by the medium flowing through. Measuring tube and medium form, together, an oscillatable system, which is, as a rule, excited to its resonance frequency. The resulting oscillatory motion of the measuring tube is usually registered by two oscillation sensors arranged on the measuring tube. The sensor signals of the oscillation sensors are accepted and conditioned by means of an input circuit. On the basis of the conditioned sensor signals, the mass flow is determined. The sensor signals have a frequency, which is equal to a frequency of the oscillation of the measuring tube. These signals are, however, shifted in phase relative to one another. The phase shift is a measure of the mass flow of the medium in the measuring tube.
EP-A 1 298 421 describes such a mass flow measurement pickup, in which the sensor signals are, in each case, fed to an associated input branch of the input circuit, where they are amplified and then digitized. The amplified, digitized sensor signals are fed to a digital signal processor, which, on the basis of these signals, determines mass flow. Determining mass flow occurs according to known methods, in which e.g. a phase shift or a time shift between the two sensor signals is calculated and the mass flow determined therefrom.
In such case, amplifiers, analog-digital converters, and passive filters possibly present in the input branches produce, as a rule, insufficiently specified and drifting phase rotations, or shifts. A zero-point error resulting therefrom is especially critical, when the determining of mass flow is done on the basis of a parallel evaluation of the separate sensor signals. While this zero-point error can be compensated, at least partially, by using sum and/or difference signals of the sensor signals, in the case of a parallel evaluation, every asymmetry of the input branches of the input circuit contributes to a resulting zero-point error.
This situation results in a measurement accuracy for the determining of mass flow, which, as a rule, can only be improved by cumbersome calibrations.